Furnace controller for available heat



March 7, 1950 H. c. M RAE 2,4 9, 6

FURNACE CONTROLLER FOR AVAILABLE HEAT Fild Nov. 15, 1946 2 Sheets-Sheet 1 INVENTOR Harm? (2 MVP/'5 151 /%,7% wwamm March 7, 1950 H, 0, om: 2,499,964

FURNACE CONTROLLER FOR AVAILABLE HEAT Filed Nov. 15, 1946 2 Sheets-Sheet? FLUE GAS TEMP. DEGREES FAHR.

GROSS FUEL INCREASE FACTOR d FLUE GAS TEMP. DEGREES FAHR.

FRACTION OF GROSS FUEL INPUT MADE AVAILABLE I j //VVNTOR Lfl- Hams'R a. Mme

v ATTORNEYS Patented Mar. 7, 1950 UNITED STATES P greater FURNACE CONTROLLER FOR AVAILABLE HEAT Application November 15, 1946, Serial No. 710,002

3 Claims.

This invention relates to combustion control, and more particularly to the control of the supply of gaseous or liquid fuel, or fuel and air mixture, to the fire box or combustion chamber of a furnace or other apparatus in which it is desirable to produce or develop useful or usable heat energy for any industrial or other purpose or process.

One object of the invention is to provide an improved control system or apparatus in which, at any temperature, high or low, of the walls of or Within the combustion chamber, upon any call or requirement for more heat, the rate of flow of fuel, or fuel and air mixture, is increased not only in direct proportion to the increased demand, but also with due regard for the heat units which are actually made available or useful out of the increased supply.

Another object is to provide an improved control system of the character described, which takes full account of variations, at different temperatures of the walls of or within the combustion chamber, in the efficiency of conversion of the B. t. u. input into available or useful heat energy.

Still another object is to provide a control system of the character described which, upon any demand for change in the rate of B. t. u. input, is sensitive not only to a condition which varies generally in step with such demands, such as fire box temperature, water or steam temperature or flow, or the like, but also is sensitive to the flue or stack temperature and thus to stack loss.

Further objects of the invention in part are obvious and in part will appear more in detail hereinafter.

In the drawings, Fig. 1 represents diagrammatically one form or arrangement of control system or apparatus embodying the invention; and

Figs. 2 and 3 are diagrams illustrating certain conditions encountered in the operation of control systems of the kind here involved.

It has long been recognized that ordinary automatic control of the B. t. u. input to a furnace or combustion chamber fails to compensate for variations in the combustion process at different temperatures in the combustion chamber. The heat actually developed, given up or made useful from the combustion of gas or liquid fuel in a fire box always is short of the possible total by the value of those heat units lost through the stack flue. As the flue temperature rises the total heat loss increases. But the important point is that the rate of increase in stack loss or available in the fire box.

is not uniform. It varies with differences in temperature. As the temperature of the Walls of and within the combustion chamber rises, efliciency decreases and the rate of stack loss and of rise in flue temperature proportionately increase.

To put it in another way, the heat arising from fuel combustion is only partly released or made available or useful in the combustion chamber. The higher its wall temperature and the higher the fuel (or fuel and air mixture) input, the higher is the stack or exit flue gas temperature. Load size and temperature also affect the flue or stack temperature. But in general it may be said that in any combustion chamber a given increment in fuel input cannot be depended upon to always yield the same percentage of available or useful heat. The actual percentage made available depends considerably upon flue or stack temperatures.

Fig. 2 of the drawings illustrates a curve, which plots flue temperatures against the percentage of available fuel increase or decrease necessary to maintain a given temperature, while the curve in Fig. 3 is the reciprocal of that in Fig. 2, plotting at various flue gas temperatures that fraction of gross fuel input which is actually made useful For instance, referring to Figs. 2 and 3, it will be noted that at 3000 flue gas temperature as measured at the thermocouple 64, there is only twenty per cent of the gross fuel input available to heat the chamber I 2. In other words, eighty per cent of the heat at this temperature is going up the stack out the exit flue l3. At about 1900" flue gas temperature, there is fifty per cent of the gross fuel input available for heating the chamber 112 and fifty per cent will escape to the stack out the flue l3. This is the phenomenon which applicant refers to in the claims as percentage of gross fuel supplied and available to increase said chamber temperature as a function of said exit flue temperature. The resistance 47 of Fig. 1 is wound to the curve of Figs. 2 and 3 so that if the furnace control thermocouple It calls for more fuel at 3000 flue gas temperature. then more gas will be supplied through line I 5 than if an equal call for increase of temperature was made at 1900 flue gas temperature. The arrangement shown at 50, 5!, 62, 83 and 6 provides a power connection with the temperature responsive device 5d constructed and arranged to cause the contact 50 to follow the temperature changes of the exit flue temperature. From these curves it will be apparent that to maintain uniform any desired fire box temperature,

a given demand for more heat costs more, in the way of increased fuel supply, to produce a given correcting efiect at a high temperature than it costs at a lower temperature.

The present system of control has been created for the purpose of introducing into the correcting effect appropriate compensation for the curved line variable referred to. Compensation for the purpose may be introduced into a wide variety of control systems, the invention not being limited to the particular form shown, except as within the scope of the claims appended hereto.

Referring now to Fig. 1, I have illustrated conventionally a furnace ID provided with walls H forming a combustion chamber or fire box l2 in which the fuel, either gas or liquid, or a fuel and air mixture, is burned, the products of combustion flowing through the fiue l3 to the stack (not shown) as is usual. Fuel, or the fuel and air mixture, is supplied through burner l4 fed by conduit l5 in which is any suitable adjustable valve l8 operated automatically by any suitable servomotor, either electric, electromagnetic, fluid pressure, mechanical, or otherwise. The drawings show for the purpose, reversible electric motor I! connected by shaft iii to the valve stem. This motor i! may be regarded as the final correct ng operator of the system, since it alone modifies or changes rate of fuel flow by its adjustment of valve l6 and is responsive or sensitive to all conditions or operations incident to or involved in a demand for change and the consequent compensating correction or change.

Since, in the usual manner of operation of furnace ID, the purpose or intent is to maintain uniform the temperature within the fire box or combustion chamber, the control system must and does include some device sensitive to and variably affected by variations in fire box temperature or some other condition which varies in step with variations in fire box temperature. Variations in water temperature, steam temperature or steam flow (as in the case of a boiler) might be utilized, or variations in fiuid pressure, as of steam or in a Bourdon tube, but for purposes of illustration, and in no sense of limitation, I show a thermocouple l9, connected to circuit wires 20, 2|, said couple extending into the combustion chamber and therefore being sensitive to variations in its temperature and producing current flow in the circuit wires 20, 2!, proportional to temperature variations. Fig. 1 is diagrammatic. Thermocouple I9 is not close enough to burner l4 to measure flame temperature, but is subject to the ambient temperature in chamber 1 2.

Since thermocouple I9 is directly responsive to variations in the condition to be maintained, here the combustion chamber temperature, it may be said to be the primary controller, and because its eifect is electrical I utilize an electric relay system for translating the impulse to which it is sensitive into adjustment or actuation of the final correcting element, valve "5.

The relay system shown for purposes of illustration includes a primary circuit, of which the thermocouple 19 forms a part, and a secondary or final circuit, sensitive to or controlled by the primary circuit and effective upon the final correcting operator, motor IT.

The system also includes suitable means for applying to motor l1, and hence to the valve 6, a modifying or compensating effect, so arranged as to take account of the curved line variable before referred to, to wit, the variations in emciency of conversion of B. t. u. input into heat units actually available or useful in the combustion chamber at different temperatures or at different rates of input, together with means for measuring or checking the final effect to insure that the ultimate adjustment and final position of valve l6 are correct. These modifying and measuring effects, in the present system, are applied to the secondary or final circuit, although that is not essential.

As shown, conductor 20 is connected through the coil 22a of a galvanometer 22 to a wire 23 connected to contact 24 of an adjustable resistance 25 provided with a slide wire 26 for cooperation with a contact 21 connected to wire 2i. The adjustable pointer or switch element 28 of the galvanometer is in circuit with a lead wire 29 from L2 and cooperates with two contacts 30, 3| connected by wires 32, 33 to motor 34, one terminal of which is connected by wires 35 to L1. Motor 34 is a reversible motor, turning in one direction or the other when the element 28 engages one or the other of contacts 30, 3|.

The shaft 36 of motor 34 not only turns the slide Wire 26 and resistance 25, but is also connected to and similarly operates, a corresponding measuring resistance 31 and slide wire 38 cooperating with contacts 39, 40 in wire 4!, forming one leg of a Wheatstone bridge, marked generally W.

An opposite leg 42 of the bridge includes a fixed or adjustable resistance 43, while the other two legs 44, 45 include respectively a resistance 46 wound and variable according to the square root law of flow, with the winding varied to produce any necessary or desirable predetermined flow pattern depending upon the particular installation and its flow characteristics, such as a straight line curve, a curve convex upwardly or downwardly or complex, and a variable resistance 41 wound to correspond to the variable curve of Fig. 2.

The bridge wire 48 includes the coil 49a of a galvanometer 49, the switch arm 50 of which cooperates with two contacts 5|, 52 connected by wires 53, 54 to the motor, a terminal of which is connected by wire 55 to L2. Member 50 is connected by wire 56 to L1.

Motor [1 is reversible, turning in one direction or the other, to open or close valve l6, according to the direction of flow in bridge wire 48 and the closing of switch arm 50 with either of contacts 5|, 52.

The variable resistance 46 is controlled or adjusted by a mercury filled manometer 51 having a pressure tube 58 connected with the fuel supply pipe l5 in such manner that as the flow in said pipe changes one leg of the mercury column rises or falls in tube 59 and shorts out more or less of the resistance 46, which dips into the mercury. The arrangement conventionally shown is but one of many suitable arrangements of electrical apparatus for measuring or determining variations in pressure or flow and reflecting the same into the relay control system.

The resistance 41, wound to correspond to the variable curve of Fig. 2, is varied by adjustment in one direction or the other of slide-wire contact arm 60, connected in leg 45 and actuated by increase or decrease in current flow through the coil 6| connected by lead wires 62, 63 to a thermocouple 64 extending into the exit or stack gas fiue 13. This thermocouple may be of the high velocity type so as to read flue temperatures more accurately. Thermocouple 64 is so far removed from chamber l2 that it measures the temperature of the gases in the exit flue 13 after they have given up as much heat as possible to chamber i2 and its contents. It measures what is commonly referred to as flue gas temperature.

The value of resistance 43 is a matter of computation. It should be adjusted to or set at such a value that when the instrument is in balance, the product of this resistance times that in the pressure leg 4 always equals the control resistance in leg 4! times the flue temperature resistance in leg 45. A suitable resistance for the illustrated example is 38 ohms.

The operation is as follows:

Let us assume that the furnace is operating at some definite B. t. u. input, resulting in definite given temperatures in the combustion chamber or fire box I2 and the flue or stack pipe 13, The control system is now in equilibrium. Both motors I1, 34 are stationary. Pointer switch members 5H, 28 are in intermediate or idle positions.

Let us now assume that for one reason or another the temperature in the combustion chamber begins to fall, thus initiating a demand for more fuel, or fuel and air mixture, and heat, to maintain the desirable temperature condition. Drop in temperature in the combustion chamber is reflected by the thermocouple I9 into the circuit including the galvanometer coil 22a, resistance 25 and slide wire 28, until the pointer switch 28 engages one or the other of the contacts 38, 3|. Thereupon motor 34 begins to turn in the proper direction, simultaneously varying the resistances 25 and 31. In this manner resistance 25 is varied sufficiently to ultimately restore the balance in the primary thermocouple circuit and return the switch member 28 to its neutral original position, thus stopping the motor.

However, the impulse thus compensated for in the primary circuithas been introduced into the secondary relay or control circuit at the Wheatstone bridge, by variation in resistance 31. This upsets the balance of the Wheatstone bridge, with a consequent flow of current in bridge wire 48, the effect of which is to move the pointer switch 50 in one direction or the other until it engages one of the contacts 5|, 52. This completes one of the circuits to the motor l1, causing it to rotate in one direction or the other and accomplish an adjustment of the valve 16, in this case opening said valve to increase the flow of fuel. If the thermostat I9 is quickly responsive to an increase in fuel supply at burner M, the fundamental type of control is provided which is the basis upon which I build my improvement. Change in thermostat 19 would again unbalance galvanometer 22 and cause a change in resistance 31 to balance the Wheatstone bridge al though there might be some hunting. Any such modulating control wherein motor I1 and valve I6 follow changes in thermostat 59 will furnish the basis underlying my improvement.

In the disclosed embodiment, adjustment of the valve l6 continues until the effect thereof, to wit, increase of fuel flow, is quickly reflected back to the Wheatstone bridge circuit by the manometer 51, which adjusts the resistance #6. This effect modifies the total effect of the bridge upon the switch arm 50, the correcting influence going on until, ultimately, switch 50 resumes its original neutral position, stopping the motor. Thermostat l9 may call for a increase in temperature and valve It may open 10% but, because of valve structure and piping, the fuel increase may not be 10%. In such a case, the

manometer 51 measures the actual increase in fuel and moves valve IS the necessary amount.

During the process, of course, the temperature increases in the flue or stack pipe 13, with a consequent effect upon thermocouple 64, which results in adjustment of arm '60 to vary the resistance 41, the latter being wound to correspond to the variable curve of Fig. 2 and thus taking account of variations in the stack flue temperature and thus of the efficiency in the conversion into useful or available heat units of any variation in the B. t. u. input.

Actually, of course, the various influences re-' ferred to do not occur serially or in order, as described, such as a first effect at thermocouple I9 producing a correcting influence at the resistance 31, followed by adjustment of pointer and operation of the motor, followed by checking or comparison of required change in flow with actual change in flow at the manometer 51, and ultimately involving modification of the correcting influence as the result of variation in stack temperature at the thermocouple 64. All of these correcting influences go on more or less simultaneously whenever a change occurs or a variation in flow is demanded. True, temperature change at thermocouple I9, resulting in adjustment of the valve [6 to a new position, may be almost instantaneously checked against or compared with the actual change in flow by a prompt effect upon the manometer 51, whereas, the resultant change in temperature at the flue may be more tardy in its effect at the thermocouple 64. But in any event, the several influences are always imparted to the control system as a whole, being cumulative in the sense that ultimately, after any demand for change, the entire system is restored to balance, with'the motor at rest and the valve in a new position, until another change is desired or required, when a similar cycle of operations is initiated and completed.

The system described, of course, is equally effective, within a reasonable range, at all furnace temperatures, but, to meet any given demand for change in the rate of B, t. u. input at different temperatures, the system supplies varying increments of additional, or of less fuel, according to the temperature at which the furnace is operating. The system, therefore, is responsive not only to combustion chamber temperature or heat given to the charge or to the doing of useful work, but also to exit flue temperature after it has given up heat to the charge and thus to the efiiciency in the conversion of B. t. u. input in terms of actual usable or available heat units.

The curves of Figs. 2 and 3 are calculated on the basis of natural gas used in the vicinity of Cleveland, Ohio, but they are probably correct within a few per cent for other fuels. They should be recalculated or corrected, however, for I preheated gas or air, because this increases the available input of B. t. u.

Other advantages of the invention will be apparent to those skilled in the art.

What I claim is:

1. Apparatus for controlling the supply of fuel to combustion heating apparatus having a heated chamber and having an exit flue for the products of combustion, comprising a fuel supply line leading to said heating apparatus, a fuel control device in said supply line, an electrical operator for said device, an electrical supply circuit for said operator including a Wheatstone bridge controlling theposition of said operator and having in one leg of said bridge a variable resistance wound to correspond to a curve of percentage of gross fuel supplied and available to increase said chamber temperature as a function of said exit flue temperature, a temperature responsive device in said exit flue, a slide-wire contact operatively engaging said variable resistance, and an actuator for said slide-wire contact having a power connection with said temperature responsive device constructed and arranged to cause said contact to follow temperature changes of said temperature responsive device.

2. Apparatus for controlling the supply of fuel to combustion heating apparatus having a. heated chamber and having an exit flue for the products of combustion, comprising a fuel supply line leading to said heating apparatus, a fuel control device in said supply line, an electrical operator for said device, an electrical supply circuit for said operator controlling the position thereof and including an element varying the amount of power supplied according to a curve of percentage of gross fuel supplied and available to increase said chamber temperature as a function of said exit flue temperature, a temperature responsive device in said exit flue, and an actuator for said power varying element having a power connection with said temperature responsive device constructed and arranged to cause said actuator to follow temperature changes of said temperature responsive device.

' 3. Apparatus for controlling the supply of fuel to combustion heating apparatus having a heated chamber and having an exit flue for the products of combustion, comprising a fuel supply line leading to said heating apparatus, a fuel control device in said supply line, a power operator controlling the position of said device, a power supply circuit for said operator including an element varying the amount of power supplied according to a curve of percentage of gross fuel supplied and available to increase said chamber temperature as a function of said exit flue temperature, a temperature responsive device in said exit flue, and an actuator for said power varying element having a power connection with said temperature responsive device constructed and arranged to cause said actuator to follow temperature changes of said temperature responsive device.

HOMER C. McRAE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,578,280 Gibson Mar. 30, 1926 1,650,623 Geissinger Nov. 29, 1927 2,159,971 Krogh May 30, 1939 2,261,343 De Florez Nov. 4, 1941 2,413,128 Willis Dec. 24, 1946 

